A Ukrainian army serviceman looks through binoculars as he stands guard at a checkpoint near the town of Debaltseve, in the Donetsk region on Saturday.

Valentyn Ogirenko/Reuters

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Moscow — Russia's military has launched a week of war games near Ukraine's border, including almost 100 bombers and strike fighters. Combined with a reported buildup of Russian troops in border regions near the rebel cities of Donetsk and Luhansk, the overwhelming display of Russian military might has revived fears that Moscow may be planning to invade eastern Ukraine.

But Russian experts say the exercise is a tactic designed to discombobulate Ukrainian forces in the war of nerves between Moscow and Kiev. And experts in both regions say that nerves may indeed be fraying in Ukraine, which has seen increasing signs of war-weariness – including a reported defection of 400 Ukrainian troops over the weekend.

Russian experts respond that August is typically a month of major military drills in Russia, and that the reported buildup of Russian troops that Kiev and NATO are complaining about this time – numbering around 15,000 – are actually far fewer than the 40,000 massed in the same area the last time there was a big invasion scare in March.

"It is true that military exercises present a very good cover for invasion preparations. You can concentrate forces, assess the potential battlefield, and put men and equipment into position," says Alexander Golts, military expert with the independent online journal Yezhednevny Zhurnal.

"After the annexation of Crimea, we cannot be completely certain that Russian leaders are acting on a rational calculus. Nevertheless, the situation does not look terribly abnormal. There are no obvious signs that Russia is preparing to use direct military force in Ukraine," he adds.

But Ukrainian experts say that the plight of the 79th Airborne Brigade, whose survivors fled across the Russian border, was an anomaly. The troops had been trapped between rebel-held territory and the Russian border, were taking fire from both directions, and had not been resupplied for many days.

"It's a defeat for Ukrainian forces, and it happened because we are not yet able to seal the Russian border," says Victor Nebozhenko, director of Ukrainian Barometer, an independent Kiev-based think tank. "This is not indicative of the general trend of things" in Ukraine's military operation in rebel-held territories, he says.

The increasingly desperate situation of the 79th brigade led some of the soldiers' wives and mothers in the unit's home town of Mykolaiv to protest against Kiev's failure to help their men last week, as reported by the Daily Beast's Anna Nemtsova.

Ukrainian experts say there are some serious problems, and they are difficult to conceal in the age of social media. "There are some military units that are highly motivated, there are others that are not so much," says Mikhail Pogrebinsky, director of the independent Kiev Center for Political Studies. "But in the case of the 79th brigade, it seems that they were actually ordered to escape to Russia after they ran out of options."

Ria Novosti reported that the troops have sought refugee status, but the Ukrainian government denies that.

War weariness

Mr. Pogrebinsky says it's difficult to assess the overall morale of Ukrainian forces fighting in the east. "But, on the whole, the mood in the country seems more pro-war than anti-war right now," he adds.

Vadim Karasyov, director of the independent Institute of Global Strategies in Kiev, says a certain battle fatigue is becoming evident in Ukrainian society.

"The first wave of military euphoria is dying down. The military solution doesn't seem to be appearing," he says. "We do not have a strong leadership that can guarantee extended mobilization, such as the Russians had through two wars in Chechnya – and even they lost the first one. The economy is not geared for war, the state is near bankruptcy, and people are confused about how it's all going to end."

Most Russian experts say they believe that Moscow is likely to continue on its present course of stoking the fires of rebellion in east Ukraine, while avoiding direct intervention and waiting for the accumulating economic and political strains of war to force Kiev to the bargaining table.

Mr. Golts says that the optimism expressed in Kiev that the war will end soon with the liberation of Donetsk and Luhansk is probably unfounded.

"The Ukrainian operation is rapidly starting to look like a classic counterinsurgency war, where the rebels can blend in with the civilian population and strike from behind," he says. "Such wars usually turn out to be unwinnable, and the rebels prevail just by continuing to exist. Think of Vietnam, our Afghanistan, your Afghanistan, or Iraq. Even the most modern, well-prepared armies find it difficult to sustain operations all the way to victory. And Ukraine's Army is far from the most capable one to try it."